I read following in a web site (http://realphototips.wordpress.com/tag/ai-servo/)
Lenses with maximum apertures of f2.8 allow the camera to use all high precision sensors. In low light or other situations that are hostile to autofocus, that’s a big deal. Lenses with a maximum aperture of f4.0 use only the center focus sensor in its “high precision” mode, and use the other sensors in their “horizontal line” only mode. Lenses with a maximum aperture of f5.6 use all sensors in their “horizontal-only” mode, and lenses with a maximum aperture of f8 use only the center sensor point, and that with horizontal sensitivity only.
I want to understand if this is completely true? I am raising this because recently I have purchased a Nikon 70-200 f/4 lens and would like to understand if I have made a wise investment, or, should I collect some more money and buy a 70-200 f/2.8 VR-II lens.
Answer
When the camera is focussing the lens, the lens is wide open, the aperture is only closed down to the selected setting when you actually close the shutter. So, based on that, an F/2.8 lens will let in twice as much light for autofocusing as the F/4 when wide open.
Did you make a mistake? Possibly not. The Nikon F/4 variants of the F/2.8 lenses are very good and, on modern Nikon bodies, don't suffer from autofocus performance issues at F/4. If you have an older Nikon body, maybe, but if you upgrade your camera at some point, the issue goes away anyways and you still have a nice lens.
Net effect, don't worry too much and enjoy the lens. You might also want to mention what Nikon body you have. :)
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